Erasure in migration imaging system

ABSTRACT

A reversal migration imaging system comprising erasing imaged migration imaging members and fabricating, layer configuration migration imaging members is disclosed.

This is a continuation of application Ser. No. 454,515, filed Mar. 25,1974, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to migration imaging systems and morespecifically, to a process of reversal migration imaging whereby animaged migration imaging member may be fabricated.

A migration imaging system capable of producing high quality images ofhigh density, continuous tone and high resolution has been developed.Such imaging systems are disclosed in copending U.S. applications Ser.No. 837,780 now U.S. Pat. No. 3,975,195 and Ser. No. 837,591 now U.S.Pat. No. 4,013,462, both filed June 30, 1969, both of which the entirecontents of which are hereby incorporated herein by reference. In atypical embodiment of these migration imaging systems, an imaging membercomprising a substrate, a layer of softenable material containingelectrically photosensitive migration material is latently imaged, e.g.,by electrically charging the member and exposing the charged member to apattern of activating electromagnetic radiation, such as light. When thephotosensitive migration material is originally in the form of afracturable layer located at the upper surface of the softenablematerial, particles of the migration material in the exposed areas ofthe migration member migrate toward the substrate when the member isdeveloped by decreasing the resistance of the softenable layersufficient to allow migration of the migration material in depth in thesoftenable material.

Various methods for developing, i.e., reducing the resistance of thesoftenable material to migration of the migration material, the latentimage in migration imaging systems are known. These various developmentmodes include solvent wash-away and softening the softenable material,e.g., by solvent vapors softening, heat softening and combinationsthereof, as well as other methods of reducing the resistance of thesoftenable material to allow migration of the migration material indepth in the softenable material.

In the solvent wash-away development method, migration material migratesin imagewise configuration in depth in the softenable layer as thesoftenable layer is dissolved, leaving an image of migrated particlescorresponding to the desired image pattern on a substrate, with thesoftenable layer and umigrated migration material substantiallycompletely washed away.

The imaging system disclosed in copending application Ser. No. 460,377,filed June 1, 1965 now U.S. Pat. No. 3,520,681, the entire contents ofwhich is hereby incorporated by reference, generally comprises acombination of process steps which include forming a latent image on amigration imaging member and developing with solvent liquid or vapor orheat or combinations thereof to render the latent image visible. Incertain methods of forming the latent image, nonphotosensitive orphotosensitively inert, fracturable layers and particulate material maybe used to form images, as described in copending application Ser. No.483,675, filed Aug. 30, 1965 now U.S. Pat. No. 3,656,990, the entirecontents of which is hereby incorporated herein by reference, wherein alatent image is formed by a wide variety of methods including chargingin imagewise configuration through the use of a mask or stencil; firstforming such a charge pattern on a separate photoconductive insulatinglayer according to conventional xerographic reproduction techniques andthen transferring this charge pattern to the imaging member by bringingthe two layers into very close proximity and utilizing breakdowntechniques as described, for example, in Carlson, U.S. Pat. No.2,982,647 and Walkup, U.S. Pat. Nos. 2,825,814 and 2,937,943. Inaddition, charge patterns conforming to selected, shaped electrodes orcombinations of electrodes may be formed by the "TESI" dischargetechnique as more fully disclosed in Schwertz, U.S. Pat. Nos. 3,023,731and 2,919,967 or by the techniques discribed in Walkup, U.S. Pat. Nos.3,001,848 and 3,001,849 as well as by electron beam recordingtechniques, for example, as disclosed in Glenn, U.S. Pat. No. 3,113,179.

Once a migration imaging member has been developed, i.e., the resistanceof the softenable material reduced sufficiently to allow migration ofthe migration material in depth in the softenable material, and themigration material has, in fact, migrated, then there is no knowntechnique for erasing this image.

There has recently been discovered a system which overcomes this problemof erasing imaged migration imaging members.

Furthermore, the fabrication of layer configuration migration imagingmembers, i.e., the placing of the migration layer material on orembedded in the surface of the softenable material, has beenaccomplished by various techniques which include dip coating, rollcoating, gravure coating, vacuum evaporation, and other techniques.However, the instant invention overcomes many of the disadvantages offorming, i.e., fabricating, layer configuration migration imagingmembers by the use of reversal migration imaging.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide a method oferasing imaged migration imaging members by a reversal migration imagingprocess.

It is a further object of this invention to provide a method ofpreparing or fabricating layer configuration migration imaging membersby a reversal migration imaging process.

The foregoing objects and others are accomplished by providing: (1) animaging member comprising a layer of softenable material containing alayer of migration material whereby the migration material is migratedin image configuration to form an imaged migration imaging member. Areverse migration force is then applied to the member to cause theimagewise migrated migration material to migrate in the oppositedirection of the first imagewise migration of migration material back tothe layer of migration material, i.e., background, while the softenablematerial is in a sufficiently softened condition to allow migration ofthe migration material thereby erasing the migration image. (2) Animaged layer configuration migration imaging member is subjected to areverse migration imaging force while the softenable material is in asufficiently softened condition to allow migration whereby the imagewisemigrated material returns to its original position in the layer ofmigration material thereby erasing the migration image. (3) A membercomprising a substrate overcoated with a layer of softenable materialwhich contains a layer of migration material located contiguous orcontacting the softenable material-substrate interface is subjected toan imagewise migration force sufficiently to cause imagewise migrationof the migration material away from the softenable material-substrateinterface and toward the free surface of the softenable material. Thesoftenable material and imagewise migrated migration material may thenbe removed, e.g., by wash-away development, whereby a densecomplementary image of unmigrated migration material remains on thesubstrate. (4) An imaging member comprising a substrate overcoated witha layer of softenable material containing a layer of migration materialcontiguous or contacting the softenable material-substrate interfacewhereby a uniform migration force is applied to the imaging member tocause migration while the softenable material is in a sufficientlysoftened condition to allow migration of essentially the entire layer ofmigration material away from the softenable material-substrate interfaceand toward the free surface of the softenable material thereby forming anormal layer configuration migration imaging member.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention as well as other objects andfurther features thereof, reference is now made to the followingdetailed disclosure of this invention taken in conjunction woth theaccompanying drawings wherein:

FIGS. 1 are partially schematic drawings representing forming a latentimage on a layer configuration migration imaging member, developing thatmember, i.e., forming a imaged migration member and applying a reversemigration imaging force to cause the migrated migration material tomigrate back to its original position in the layer configuration whilethe softenable material is softened sufficiently to allow reversalmigration of the migration material thereby erasing the image. Theerased member is then reimaged illustrating that the member may beimaged and erased as many times as desired.

FIGS. 2 are partially schematic drawings representing an imaged layerconfiguration migration imaging member and the steps of applying areverse migration force to cause migration of the migration materialback to its original position in the layer of migration material therebyerasing the image. The erased member is then reimaged illustrating thatthe member may be imaged and erased as many times as desired.

FIGS. 3 are partially schematic drawings representing a member with alayer of migration material located at the softenable material-substrateinterface and the steps of applying an image-wise migration force tocause imagewise migration of the migration material away from thesubstrate and removing the softenable material and imagewise migratedmaterial whereby a dense comple mentary image remains on the substrate.

FIGS. 4 are partially schematic drawings representing a member where thelayer of migration material is located at the softenablematerial-substrate interface and the steps of applying a uniformmigration force to the member while the softenable material is softenedsufficiently to allow migration whereby the entire layer of migrationmaterial migrates away from the interface towards the free surface ofthe softenable material thereby forming a layer configuration migrationimaging member. This member is then imaged by normal migration imagingtechniques.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1A which shows a schematic drawing of one of theembodiments which comprises an imaging member 1 which compriseselectrically insulating softenable layer 3 which contains a layer ofmigration material 2.

Softenable layer 3, may be any suitable material, typically a plastic orthermoplastic material, which is capable of having its resistance tomigration reduced sufficiently to allow migration of the migrationmaterial in depth in the softenable material. Furthermore, as a specificpreferred embodiment of development, the softenable material should becapable of being soluble in a solvent or softenable, for example, in asolvent liquid, solvent vapor, heat or combinations thereof. Softenablelayers which allow charge injecting which results in the removal of thecoulombic migration force on the particles before reverse migration hasoccurred, are to be avoided. Therefore, the softenable material shouldbe capable of retaining an electrical or electrostatic charge, which issufficient to cause reverse migration to occur, on its surface for fromabout 0.1 to about 30 seconds subsequent to the beginning of the step ofsoftening the softenable material sufficient to allow reverse migrationto occur. Preferably, the softenable material should have a softeningrange of at least about 10° C. and initial softening point of less thanabout 90° C. and a surface melt viscosity in the range between about 10⁴to 10⁹ poise.

"Softenable" as used herein to depict softenable layer 3 is intended tomean any material which can be rendered by the development step thereofor the softening step hereof more permeable to particles migratingthrough its bulk.

Softenable materials which are to be used for softenable layer 3 arematerials which will, under certain circumstances, i.e., during reversalmigration, while the softenable material is in a soft conditionsufficient to allow reverse migration of the migration material, i.e.,the migration particles are attracted to the surface of the softenablematerial which will result in the particles migrating to a positionwhich will erase the migrated image, must be capable of retaining equaland opposite charges on its free surface, i.e., the free surface of thesoftenable material, sufficient to allow reverse migration of theoppositely charged migrated migration particles to occur.

Upon uniform charging of the free surface of the softenable material, itis preferred that the softenable material hold the charges on thesurface for a sufficient time in order for the migration particles withopposite charges to migrate up through the film toward the surface whichcontains this charge before the charge is dissipated.

Typically preferred substantially electrically insulating softenablematerial includes a host of plastic and thermoplastic material, examplesof which are specifically recited in copending application Ser. No.837,780, filed June 30, 1969, the entire contents of which is herebyincorporated by reference; paraffins and waxes and other materials whichare typically substantially electrically insulated, and capable ofhaving its resistance to migration reduced to allow migration of themigration material may be used in the advantageous system of the presentinvention. Such substantially electrically insulating softenablematerials will typically have resistivities not less than about 10¹⁰ohms-cm, and preferably have resistivities not less than about 10¹²ohms-cm.

Especially preferred substantially electrically insulating softenablematerials include copolymers of styrene and hexylmethacrylate;copolymers of styrene and n-butyl-methacrylate; copolymers of styreneand octly-acrylate; copolymers of styrene and t-decylate-styrene andcopolymers of methyl methacrylate and t-deylate-styrene.

Softenable layer 3 may be of any suitable thickness, with thicker layersgenerally allowing greater reverse migration to occur. Softenable layerthicknesses from about one-half to about 16 microns are found to bepreferred.

Softenable layer 3 is illustrated in FIG. 1B with the advantageoussurface skin layer 8 formed at one of the free surfaces of thesoftenable material. The advantageous surface skin 8 may either beformed as a substantially continuous layer at the surface of softenablelayer 3 or as a discontinuous layer, such as distinct, separate skinportions upon the softenable layer arranged, for example, as closelyspaced dots. Any suitable method may be used to form the surface skinlayer 8. Where the imaging member includes a layer of migrationmaterial, the surface skin layer 8 should be formed after the layer ofmigration material is applied at the upper surface of the softenablelayer 3. The surface skin layer 8 may typically be formed either bycoating a layer of skin material over the softenable layer 3, or bychanging the properties of the surface of the softenable layer in situ.Furthermore, surface skin 8 may be of high viscosity material which isformed on the surface of softenable layer 3 either by modification ofthe surface of the softenable layer 3 or by coating a layer of anothermaterial thereon. Alternatively, surface skin 8 may comprise a chemicalcomposition different from the bulk of softenable material 3, but bepart of the single layer of softenable material 3. Therefore, softenablematerial 3 may comprise a single layer wherein a portion of softenablelayer 3 comprises a surface skin 8 having a chemical compositiondifferent from the bulk of softenable material 3 and having a thicknessnot greater than about 0.3 micron and a viscosity during development ofthe member greater than that of the bulk of said softenable material.Furthermore, the softenable layer may contain a surface skin located onat least one entire free surface of the softenable layer formed byexposing the free surface to hardening radiation sufficiently to formsurface skin 8 having a thickness in the range not greater than about0.3 micron and having a viscosity during imaging of said member greaterthan the viscosity of the softenable material of said softenable layer3.

Typical methods for forming surface skin layer 8 include exposure toactinic light, x-rays, beta rays, gamma rays, electrical bombardment,corona charging, high voltage discharge, exposure to visible light,exposure to air, contact with chemical means such as oxidizing agentsand/or linking agents, overcoating with a high viscosity material whichmigrates to the surface of the softenable layer as the softenable layercures, or any other chemical, physical or radiative means capable offorming the surface skin layer having a viscosity greater than theviscosity of the bulk of softenable layer 8.

Surface skin 8 should be substantially electrically insulating andcapable of retaining an electrical or electrostatic charge, which issufficient to cause reverse migration to occur, on its surface for about0.1 to about 30 seconds subsequent to the beginning of the softeningstep required to allow reverse migration to occur.

As mentioned above, the softenable material may be a single layer ofsoftenable material where the entire upper portion of the softenablelayer is a surface skin having a chemical composition different from thebulk of softenable material and having a thickness not greater thanabout 0.3 micron and a viscosity during imaging of said member greaterthan that of the bulk of the softenable material. The preferred surfaceskin in the above described embodiment may be any suitable material, forexample, a phenol formaldehyde resin; a mixture of polystyrene andacrylate polymers; or a phenol formaldehyde resin selected from thegroup consisting of p-tertiarybutyl phenol formaldehyde resin andp-phenyl phenol formaldehyde resin. The above listing of materials ismerely a representative example of the preferred materials which may beused as surface skins. These materials should be substantiallyelectrically insulating and capable of retaining an electrical orelectrostatic charge, which is sufficient to cause migration to occur,on its surface for about 0.1 to about 30 seconds subsequent to thebeginning of the softening of the softenable material sufficient toallow reverse migration to occur.

In various embodiments of the novel reverse migration imaging system ofthe present invention, the softenable material may be overcoated with anoverlayer. Suitable material which may be used as the overlayer istypically substantially electrically insulating which are capable ofretaining an electrostatic charge, which is sufficient to cause reversemigration to occur, on its surface for about 0.1 to about 30 secondssubsequent to the beginning of the softening step necessary to allowreverse migration to occur. The preferred overlayer material comprisesphenol formaldehyde resin; mixtures of polystyrene; mixtures ofpolystyrene and acrylic polymers. The phenol formaldehyde resin may beselected from the group consisting of p-tertiarybutyl phenolformaldehyde resin and p-phenyl phenol formaldehyde resin.

Materials suitable for surface skin are those made hardenable byexposure to hardening radiation sufficient for forming a skin having athickness of not greater than about 0.3 micron and having a viscosityduring development of the member greater than the viscosity of thesoftenable material of the softenable layer. Such radiation includesultraviolet radiation. Furthermore, for a more detailed listing ofsuitable materials, which may be used as overlayer materials, may befound in more detail in copending application Ser. No. 424,033, filedDec. 12, 1973, now abandoned, the entire contents of which is herebyincorporated by reference.

Migration layer 2, portions of which migrate towards or to the substrateduring normal migration imaging under the influence of normal migrationforces is disclosed in copending U.S. patent application Ser. No.837,780, filed June 30, 1969. Upon applying a reverse migration force tothe migrated migration material sufficient to cause migration, themigrated image pattern migrates back toward the background material whenthe softenable material is softened sufficiently to allow migrationthereby erasing the image pattern. Layer 2 may be a fracturable layer ora particulate layer. Layer 2 may be continuous or a semi-continuouslayer, such as a fracturable layer in a swiss cheese pattern, which iscapable of breaking up in discrete particles of the size of an imageelement or less during the development step permitting portions tomigrate toward the substrate in image configuration.

Typical layer configuration migration imaging members which may be usedin the instant invention, are disclosed in copending U.S. applicationSer. No. 837,780, filed June 30, 1969, the entire contents of which ishereby incorporated by reference herein.

The thickness of layer 2 is preferably from about 0.01 to about 2.0microns thick although 5 micron layers have been found to give goodresults for some materials.

When layer 2 comprises particles, a preferred average particle size isfrom about 0.01 to about 2.0 microns. Layers of particle migrationmaterial preferably should have a thickness ranging from about thethickness of the smallest element of migration material in the layer toabout twice the thickness of the largest element in that layer. Itshould be recognized that the particles may not all be packed tightlytogether laterally or vertically so that some of the thickness of layer2 may constitute softenable material.

Layer 2 may comprise any suitable material selected from an extremelybroad group of materials and mixtures thereof including electricalinsulator, electrical conductors, photosensitive materials and opticallyinert particles. For the modes hereof employing an electrical reversemigration force, the migrating portions of layer 2 should besufficiently electrically insulating to hold their electrical chargeuntil the desired amount of migration has occurred. Conductive particlesmay be used if lateral conductivity is minimized by loose packing sothat neighboring particles are in poor electrical contact.

Migration material, preferably, should be substantially insoluble in thesoftenable material and otherwise not adversely reactive therewith, andin any solvent liquid or vapor which may be used in the softening stephereof. Photosensitive materials for layer 2 permit the imaging memberhereof to be imaged and erased by the optimum electrical-optical modehereof, to be further described, which is a simple, direct, opticallysensitive method of producing high quality images which may then beerased by applying a reverse migration force, according to thisinvention. Typical such photosensitive materials include inorganic ororganic photoconductive insulating materials; materials which undergoconductive changes when photoheated, for example, see Cassiers, Photog.Sci. Engr. 4, No. 4, 199 (1960); materials which photoinject or injectwhen photoheated.

While photosensitive materials may be used in the preferred electricalreverse migration force mode, employing electrostatic forces, anysuitable non-photosensitive migration material, such as graphite, dyes,starch, garnet, iron oxide, carbon black, iron, tungsten and mixturesthereof may be used as described in copending application Ser. No.483,675, filed Aug. 30, 1965, the entire contents of which is herebyincorporated by reference herein.

While photoconductive particles, (and "photoconductive" is used in itsbroadest sense to mean particles which show increased electricalconductivity when illuminated with electromagnetic radiation and notnecessarily those which have been found to be useful in xerography inxerographic pigment-binder plate configurations) have been found to be aclass of particles useful is "electrically photosensitive" particles inthis invention and while the photoconductive effect is often sufficientin the present invention to provide an "electrically photosensitive"material, it does not appear to be a necessary effect. Apparently, thenecessary effect according to the invention is the selective relocationof charge into, within or out of the material or particles, saidrelocation being effected by light acting on the bulk or surface of the"electrically photosensitive" material, by exposing said material orparticle to activating radiation which may specifically includephotoconductive effects, photoinjection, photoemission, photochemicaleffects and others which cause said selective relocation of charge.

Referring now to the reverse imaging methods of this invention whichinclude (1) forming a latent image on layer 2 of member 1 as illustratedin FIGS. 1C and 1D and developing that member, i.e., forming an imagedmigration member, as illustrated in FIG. 1E and then applying a reversemigration imaging force, as illustrated in FIG. 1F, to cause themigrated migration material 2b to migrate back to its original positionin layer 2 while the softenable material 3 is softened sufficiently toallow reverse migration to occur thereby erasing the image asillustrated in FIG. 1G. (2) Imaged migration imaging member 9, asillustrated in FIG. 2A is subjected to a reverse migration force, asillustrated in FIG. 2C, to cause migration of the migration material 2bto its original position in the layer of migration material therebyerasing the image, as illustrated in FIG. 2D. (3) Member 10, asillustrated in FIG. 3A, where the layer of migration material is locatedat the softenable material-substrate interface 11, is subjected to animagewise migration force as illustrated in FIGS. 3C and 3D to causeimagewise migration of the migration material away from the substratethereby leaving the complementary image on the substrate, as illustratedin FIG. 3D. The member may then be wash-away developed thereby removingthe softenable material and the imagewise migrated migration asillustrated in FIG. 3E (4) Member 10, as illustrated in FIG. 4A, wherethe layer of migration material is located at the softenablematerial-substrate interface 11, is subjected to a uniform migrationforce as illustrated in FIG. 4C to cause migration of substantially theentire layer of migration material 6 away from interface 11 and towardsurface 12 of softenable layer 3 thereby forming layer configurationmigration imaging member 13, as illustrated in FIG. 4D.

Modes of imaging migration imaging members as illustrated in FIGS. 1A-1Eare taught in copending U.S. application Ser. No. 837,780, filed June30, 1969, the entire contents of which are hereby incorporated byreference herein. These modes include (1) applying to the migrationlayer material 2 an imagewise migration force, as illustrated in FIGS.1C and 1D, which typically is associated with a latent imagewise chargeon the imaging member, which causes directly or indirectly a force onthe migration layer towards the bulk of the softenable layer andtypically towards a face of the softenable layer or, where a substrateis used, towards the substrate-softenable layer interface; saidimagewise migration force applying step occurring before, during orafter the development step of reducing the resistance of the softenablematerial to migration of the migration material sufficiently to allowmigration of the migration material in depth in the softenable materialand (2) applying to the migration layer material an imagewise migrationforce during or after the development step of reducing the resistance ofthe softenable material to migration of the migration materialsufficiently to allow migration of the migration material in depth inthe softenable material to form a member, as illustrated in FIG. 1E.

There are a variety of forces which can be applied to and can be made toact on the migration layer in image configuration to cause migration ofthe migration material in depth in the softenable material, asillustrated in FIGS. 1A-1E. Such forces include electrical orelectrostatic, magnetic, gravitational, and centrifugal forces.

Modes of applying reverse migration forces to the migration layerinclude (1) applying charges opposite to those on the migrated migrationmaterial to produce an attraction of the migrated migration material tothe opposite polarity charges on the opposite surface of the softenablelayer as illustrated in FIGS. 1F, 2C, 3C and 4C; (2) applying animagewise or uniform external electric field acting on either uniformlyor imagewise charge migration material; (3) applying a uniform orimagewise magnetic field acting upon either a uniform or imagewisemagnetized migration layer; (4) applying centrifugal forces to themigration material; and (5) applying gravitational forces to themigration material. All the above mentioned forces may be applied eitherimagewise (except the latter two) or uniformly depending upon whether ornot imagewise migration is to be accomplished. Furthermore, the reversemigration force may be accomplished by imagewise softening of thesoftenable material to allow reverse migration of the migrationmaterial.

Referring now specifically to the imaging modes hereof and to FIGS.1A-1G. FIG. 1A shows a partial schematic of a layer configurationmigration imaging member 1 comprising softenable layer 3 and a layer ofmigration material 2. FIG. 1B illustrates a layer configurationmigration imaging member, comprising softenable material 3, a layer ofmigration material 2 and an overlayer of surface skin 8.

Referring now to FIG. 1C, a latent image is formed by the optimumelectrical-optical mode hereof, in member 1 where layer 2 comprisesphotosensitive materials by the preferred method comprising the steps ofuniformly charging with a corona device (FIG. 1C) and imagewise exposing(FIG. 1D). In FIG. 1C, the imaging member is uniformly electrostaticallycharged, illustratively by means of a corona discharge device 4 which isshown to be traversing the member from left to right depositing auniform, illustratively negative charge on the surface of layer 2. Forexample, corona discharge devices of the general description andgenerally operating as disclosed in Vyverberg U.S. Pat. No. 2,836,725and Walkup U.S. Pat. No. 2,777,957, have been found to be excellentsources of corona discharging devices useful in discharge of member 1.Other charging techniques and other corona discharging devices aredescribed in copending application Ser. No. 837,780, filed June 30,1969.

Referring now to FIG. 1D, a second step in the embodiment of the optimumelectrical-optical mode of forming the latent image after charging,member 1 is exposed to an imagewise pattern of activating radiation 5.For purposes of illustration, the surface electrical charges aredepicted as having moved into particulate layer 2 in the illuminatedareas. For a detailed description of more optimum processes of formingthe latent image is described in copending U.S. patent application Ser.No. 837,780.

Copending U.S. application Ser. No. 837,780, filed June 30, 1969,describes imaging systems suitable for use in the present invention ingreat detail, and all the disclosure therein and especially thedisclosure relating to such imaging processes, imaging members andmaterials suitable for use in the migration imaging members usedtherein, is hereby expressly incorporated by reference into the presentspecification. Member 1 having the electrical latent image thereon, asillustrated by FIG. 1D, is developed by softening the softenablematerial sufficient to allow migration of the migration material throughthe softenable layer 3, forming imaged migration imaging member 9, asillustrated by FIGS. 1E.

The application of heat, solvent vapors, or combinations thereof, or anyother means for softening the softenable material of softenable layer 3to allow migration of the migration material 2 may be used to developthe latently imaged member, whereby migration material 2 is allowed tomigrate in depth in softenable layer 3 in image configuration. FIG. 1Eillustrates developed member 9 where the migration material is shownmigrated as migration material 2b and the unmigrated material, i.e.,background, is illustrated as migration material 2a. These members arefully described in copending U.S. patent application Ser. No. 837,780,filed June 30, 1969.

In FIG. 1F, member 9 is uniformly charged with an opposite polarity tothat which is contained by the migrated charged migration material. Thisreverse imagewise migration force is an attraction of charged migrationmaterial 2b to charges of a polarity opposite the polarity of charges onthe migrated migration material 2b. Said opposite polarity charges aredeposited at the surface of the softenable material 3 thereby causingthe migrated migration material 2b to migrate towards the backgroundmigration material 2a upon softening softenable layer 3 sufficient toallow reverse migration of migration material 2b. When the image patternconfiguration of migration material 2b migrates to the backgroundmaterial 2a, the image pattern 2b is erased, as illustrated by FIG. 1G.

In FIG. 1H, the erased member 1, as illustrated in FIG. 1G, may belatently imaged again by the preferred method comprising the steps ofuniformly charging with a corona device 4 (FIG. 1H) and imagewiseexposed (FIG. 1I). Member 1, as illustrated in FIG. 1I, is againdeveloped, by the same modes used to develop member 1 (FIG. 1D)previously discussed, by softening the softenable material sufficient toallow migration of the migration material through the softenable layer 3forming imaged migration imaging member 9 as illustrated by FIG. 1J.This member may be erased and imaged as many times as desired.

The materials suitable for use as softenable layer 3 and migrationmarking material 2 are the same materials disclosed in aforementionedU.S. copending application Ser. No. 837,780, filed June 30, 1969, whichis incorporated herein by reference. However, the materials suitable forlayer 3 when no overlayer or surface skin is present must be capable ofretaining an electrical or electrostatic charge, i.e., a chargesufficient to cause migration to occur when the softenable material issoftened sufficient to allow migration, on its surface from about 0.1 toabout 30 seconds subsequent to the beginning of the softening step whichallows reverse migration to occur.

In various embodiments of the novel migration imaging members of thepresent invention, the migration material may be electricallyphotosensitive, photoconductive, photosensitively inert, magnetic,electricall conductive, electrically insulating, or any combination ofmaterial suitable for use in instant migration imaging systems. Althoughthe softenable layer has not been described as being coated on asubstrate in some embodiments, the softenable layer has sufficientstrength and integrity to be substantially self-supporting. However, itmay be brought into contact with a suitable substrate during the imagingprocess or may contain a substrate initially.

It should be noted that layer 8 (FIGS. 1B, 3B and 4B) which may be asurface skin or overlayer, is capable of retaining an electrostaticcharge, i.e., a charge sufficient to cause migration to occur when thesoftenable material is softened sufficient to allow migration, on itssurface from about 0.1 to about 30 seconds subsequent to the beginningof the softening step which allows reverse migration to occur. In thisembodiment, the softenable maerial 3 may be any suitable material asdescribed in copending U.S. application Ser. No. 837,780, filed June 30,1969, with no limitations as to the surface characteristics forretaining charge. Furthermore, these overcoatings will typically,appreciably soften when the migration imaging members are developed orerased by reverse migration imaging. However, in various embodiments, itmay be advantageous to use harder overlayer materials which may permitsolvent vapors to penetrate through to the softenable layers. Overlayer8 may include materials such as Bavick 11, a copolymer of alpha methylstyrene and methyl methacrylate; Mylar, a polyester resin available fromDuPont; Elvacet, a polyvinyl acetate resin available from DuPont; andothers as well as mixtures and copolymers thereof.

The overlayer 8 may also be transparent, translucent or opaque dependingupon the imaging system in which the overcoating member is desired foruse. Where the overlayer comprises substantially electrically insulatingsoftenable material, it will typically have resistivities not less thanabout 10¹⁰ ohm-cm., and preferably have resistivities of not less thanabout 10¹² ohm-cm. Overlayer 8 is typically preferably of a thickness upto about 75 microns, although thicker overlayers may be suitable anddesirable in certain embodiments. More preferably, the thickness of theoverlayer generally should range from about 0.01 to 1.0 micron. Apreferred range of thickness which yields outstanding results is fromabout 0.1 to 0.5 micron. Furthermore, the material must have the samecapacity of retaining charge on its surface as mentioned above, for thesurface skins or overlayer materials or the softenable material whichdoes not contain a surface skin or overlayer.

FIG. 2A illustrates an imaged migration imaging member, and FIG. 2Billustrates an improved migration imaging member and an overlayer onsurface skin 8 contained on softenable layer 3. FIG. 2A illustrates themigration material as migrated migration material 2b and unmigratedmigration material 2a, i.e., background.

The member, as illustrated in 2A, is uniformly charged with a charge ofthe opposite polarity of that which is contained upon the migratedparticles, i.e., migrated particles 2b, wherein a reverse migrationforce is created whereby the charged migrated migration material 2b isattracted to charges of a polarity opposite to the polarity of chargeson the migrated migration material 2b. The opposite polarity charges areat the surface of the softenable material closest to the unmigratedmigration material 2a, i.e., background, or a location spaced apart fromsaid migrated migration material 2b, which causes the migrated migrationmaterial 2b to migrate to the background migration material 2a, when thesoftenable material is softened sufficiently to allow migration, therebyerasing the imaged migration imaging member as illustrated by FIG. 2D.

The migrated migration particles 2b may contain a charge so that whenthe opposite polarity charge is placed on the surface of the member inorder to cause these particles to migrate. Alternatively, theseparticles, i.e., migrated migration particles 2b, may be charged, ifsufficient time is allowed, in situ, e.g., by the same modes used tocharge the member, i.e., particles, as illustrated in FIGS. 1C, 1D, 1H,2E, 2F, 4E and 4F. Where the migrated migration particles 2b (FIGS. 1E,1F and 2C) are photosensitive material or if the member contains asubstrate 7, as illustrated in FIGS. 3A-3D and FIGS. 4A-4D, thesubstrate may be photosensitive or where the particles arephotosensitive inert, the softenable material 3 may be photosensitive. Acorona device may uniformly electrostatically charge the free surface ofthis type of member. Techniques for charging migration material througha photosensitive substrate is disclosed in copending U.S. applicationSer. No. 837,780, filed June 30, 1969, and is hereby incorporated byreference into this application.

In FIG. 2E, the erased member 1 may be latently imaged by the preferredmethod comprising the steps of uniformly charging with a corona device 4(FIG. 2E) and imagewise exposed (FIG. 2F). Member 1, as illustrated inFIG. 2F, is developed by softening the softenable material sufficient toallow migration of the migration material through the softenable layer 3forming imaged migration imaging member 9, as illustrated in FIG. 2G.This member may be erased and imaged as many times as desired.

Referring now to FIG. 3A in which imaging member 10 is illustratedcomprising a substrate 7, a layer of substantially insulating softenablematerial 3 on said substrate 7 and said softenable material 3 containinga layer of migration material 6 contiguous or contacting interface 11 ofsaid softenable material 3 and said substrate 7, softenable material 3is capable of having its resistance to migration of migration materialdecreased sufficiently to allow migration of migration material 6 insaid softenable material 3. Illustrated in FIGS. 3A-3E is the process ofapplying a reverse imagewise migration force to migration material 6sufficient to cause imagewise migration of the migration material 6 awayfrom the interface 11 and toward free surface 12, illustrated in FIG.3C. The process of developing the imaging member 10 by decreasing theresistance of migration material 6 in depth in the softenable layer 3 atleast sufficient to allow imagewise migration away from interface 11 ofsoftenable material 3 and substrate 7 toward free surface 12 ofsoftenable material 3, is illustrated in FIG. 3D. If softeningdevelopment is used, i.e., softening of the softenable materialsufficient to allow migration of the migration material, softenablematerial 3 and imagewise migrated material 6b will remain on the member,as illustrated in FIG. 3D. However, if wash-away development is used,i.e., a solvent applied which dissolves softenable material 3, thesoftenable material will be dissolved and the softenable material alongwith the imagewise migrated material 6b will be washed away from themember resulting in unmigrated migration material 6a remaining on thesubstrate 7 in complementary image configuration, as illustrated in FIG.3E.

Substrate 7 may be electrically conductive or insulating. Also,substrate 7 may be photosensitive or non-photosensitive. Conductivesubstrates generally facilitate the charging or sensitizing of themember and typically may be of copper, brass, nickel, zinc, chromium,stainless steel, conductive plastics and rubbers, aluminum, steel,cadium, silver, gold or paper rendered conductive by the inclusion of asuitable chemical therein or through conditioning in a humid atmosphereto insure the presence therein of sufficient water content to render thematerial conductive. Suitable substrates are disclosed in copending U.S.application Ser. No. 837,780, filed June 30, 1969.

The reverse migration force may be applied by any number of knowntechniques. In the preferred modes an imagewise electrical orelectrostatic charge is applied to surface 12, as illustrated in FIG.3C, by charging through a stencil or a shaped electrode, etc. Surface 12of softenable material 3 of FIG. 3C and 3D must be capable of retainingan electrical or electrostatic charge, i.e., a charge sufficient tocause migration to occur when the softenable material is softenedsufficient to allow migration, on its surface from about 0.1 to about 30seconds subsequent to the beginning of the softening step which allowsreverse migration to occur.

The layer of migration material comprising particles 6, illustrated inFIG. 3A, may contain a charge so that when the opposite polarity chargeis placed on surface 12 of member 10, illustrated in FIG. 3A, particles6 will migrate toward surface 12 when the softenable material issoftened sufficient to allow migration of particles 6. Alternatively,particles 6 may be charged, if sufficient time is allowed, in situ,e.g., by the same modes used to charge members as illustrated in FIGS.1C, 1D, 1H, 1I, 2E, 2F, 4E and 4F.

Member 10 of FIG. 3D is developed, i.e., the softenable materialresistance to migration of the migration material is reducedsufficiently to allow migration of the migration material 6, therebyforming the member as illustrated in FIG. 3D. As mentioned, this membermay be wash-away developed to form the member as illustrated in FIG. 3E.Alternatively, wash-away development may be applied to the membersubsequent the member being subjected to an imagewise reverse migrationforce thereby dissolving the softenable material 3 and removing thismaterial along with imagewise migrated migration material 6b. Theresultant member is a dense, high compacted excellent complementaryimage adhered to substrate 7 which may be more permanently fixed byknown fixing techniques, such as disclosed in copending U.S. patentapplication Ser. No. 168,739, filed Aug. 3, 1971, the entire contents ofwhich is hereby incorporated by reference herein.

Referring now to FIGS. 4A-4D which illustrate a method of preparingmigration imaging member 13, as illustrated in FIG. 4D. FIGS. 4A-4Cillustrate a member 10 comprising substrate 7 and a layer ofsubstantially insulating softenable material 3 on substrate 7.Softenable material 3 contains a layer of migration material 6contiguous or contacting interface 11 of softenable material 3 andsbustrate 7. Softenable material 3 is also capable of being softenedsufficiently to allow migration of the migration material 6 insoftenable layer 3. FIG. 4C illustrates the process of applying areverse migration force to the migration material 6 sufficient to causemigration of substantially the entire layer of migration material 6 awayfrom the substrate 7 and towards the free surface 12 of the imagingmember 10 and softening softenable layer 3 sufficient to allow migrationof substantially the entire layer of migration material 6 away from thesubstrate 7 and towards free surface 12 of the softenable material 3whereby member 13, as illustrated in FIG. 4D, is fabricated.

FIG. 4B illustrates the member 10 of FIG. 4A, additionally comprising anoverlayer or surface skin 8. The overlayer or surface skin 8 may be ofthe same material as described earlier for use in member 1, illustratedin FIG. 1 or member 9, as illustrated in FIG. 2B or member 10, asillustrated in FIG. 3B. Member 10, as illustrated in FIG. 4C, may beuniformly charged with a corona charging device 4 in the same manner asdescribed for charging member 1, illustrated in FIG. 1C.

Member 10 may be softened by the same methods as described for softeningmember 1, as illustrated in FIG. 1E and 1F and FIG. 2C.

The layer of migration material 6 may contain a charge or may be chargedin situ as described for charging in FIGS. 2A-2C and FIGS. 3A-3C. Uponsoftening softenable material 3, layer 6 migrates away from substrate 7and towards surface 12 thereby forming a normal migration imaging member13, illustrated in FIG. 4D.

In FIG. 4E, the member 1 may be latently imaged by the preferred methodcomprising the steps of uniformly charging with a corona device 4 (FIG.4E) and imagewise exposed (FIG. 4F). Member 1, as illustrated in FIG.4F, is developed by softening the softenable material sufficient toallow migration of the migration material through the softenable layer 3forming imaged migration imaging member 9, as illustrated by FIG. 4G.The member may be erased and imaged as many times as desired.

The following Examples further specifically define the present inventionreverse migration imaging system. The parts and percentages are byweight unless otherwise indicated. All exposures are from a tungstenfilament light source, unless otherwise specified. The Examples beloware intended to illustrate various preferred embodiments of the reversemigration imaging system of this invention. The Examples are directedprimarily to softening development since wash-away development is amplydescribed in Goffe, U.S. Pat. No. 3,520,681.

EXAMPLE I

An imaging member is produced by preparing a softenable materialcomprising about a 12 weight percent solution of a custom synthesizedabout 80/20 mole percent copolymer of styrene and hexylmethacrylatehaving a molecular weight of about 41/300, intrinsic viscosity intoluene at about 25° C. of about 0.16. The softenable material is thencoated onto an aluminized Mylar substrate with a gravure roller and thenallowed to dry. The softenable material has a thickness of approximately2 microns. The surface of the softenable material is then coated withamorphous selenium by the vacuum deposition process as fully describedin copending application Ser. No. 813,345, filed Apr. 3, 1969 nowabandoned. Vacuum evaporation of selenium onto the softenable layerresults in an layer of particulate selenium being formed, having averageparticle size of about 0.7 micron. This member, i.e., the surface of thesoftenable material containing the layer of particulate selenium, isthen laminated with an aluminized Mylar substrate by using a hot roller(100° C.) and then the member is allowed to cool. Then the first layerof aluminized Mylar opposite the side of the member containing the layerof selenium is stripped away thereby forming a member comprising a 2micron layer of softenable material with the selenium layer near thesoftenable layer-substrate interface.

This imaging member is uniformly electrostatically charged using acorona device to a surface potential of positive 200 volts, imagewiseexposed to activating electromagnetic radiation, here light, ofapproximately 5 ergs/cm² of 4,000 angstroms light through a photographictransparency in contact with the member. Then the member is heated to100° C. for 20 seconds. The previously exposed areas will migrate awayfrom the softenable material-substrate interface and toward the freesurface of the softenable material producing a blueish color withincreased transparency in these areas and the unmigrated particles,i.e., unexposed particles, will remain in their original positionshowing the original member's density. This results in a contrastdensity of approximately 1.

EXAMPLE II

An imaging member is prepared, charged and exposed as in Example I.

The member is then developed by immersing for a few seconds in1,1,1-trichloroethane. The migrated particles and substantially all ofthe softenable material is wahed away thereby leaving the complementaryimage on the substrate.

EXAMPLE III

An imaging member is prepared as described in Example I. In addition, a1 mil Mylar layer is laminated to the free surface of the softenablematerial, opposite the substrate, as an overlayer of material. Themember is then charged positive to a voltage which resulted in a fieldof 100 volts/micron. The member is imagewise exposed and heated, i.e.,developed, as in Example I. An image is observed as described in ExampleI.

EXAMPLE IV

An imaging member is produced by preparing a softenable materialcomprising about a 12 weight percent solution of a custom synthesizedabout 80/20 mole percent copolymer of styrene and hexylmethacrylatehaving a molecular weight of about 41/300, intrinsic viscosity intoluene at about 25° C. of about 0.16. The softenable material is thencoated onto an aluminized Mylar substrate with a gravure roller and thenallowed to dry. The softenable material has a thickness of approximately1.15 microns. The surface of the softenable material is then coated withamorphous selenium by the vacuum deposition process as fully describedin copending application Ser. No. 813,345, filed Apr. 3, 1969. Vacuumevaporation of selenium onto the softenable layer results in an layer ofparticulate selenium being formed, having average particle size of about0.25 micron.

The member is uniformly negatively charged to 100 volts and uniformlyexposed to activating electromagnetic radiation of approximtely 10ergs/cm² of 4,000 angstroms light. The member is then exposed to vaporsof Freon 113, trichlorotrifluoroethane, available from E. I. duPont deNemours and Co., for 30 seconds where upon all the particles migratecompletely to the substrate and form the original member's density.

EXAMPLE V

The member as fabricated in Example IV, i.e., a member comprising alayer of softenable material overlying a substrate and said softenablematerial containing a layer of selenium at the softenablematerial-substrate interface, is imagewise positively charged with acorona charging device through a mask to positive 100 volts. The memberis then exposed to vapors of Freon 113 for 30 seconds where upon theparticles migrate away from the interface and to the free surface of thesoftenable material in image configuration and repack to form theoriginal member's density in image configuraion.

EXAMPLE VI

The member as fabricated in Example IV, i.e., a member comprising alayer of softenable material overlying a substrate and said softenablematerial containing a layer of selenium at the softenablematerial-substrate interface, is uniformly positively charged to 100volts and then exposed to vapors of Freon 113, trichlorotrifluoroethane,available from E. I. duPont de Nemours and Co., for 30 seconds whereupon all the particles uniformly migrate away from the interface andtowards the free surface of the softenable material.

The member is then imagewise recharged negatively to 100 volts andblanket exposed to activating electromagnetic radiation of 10 ergs/cm²at 4,000 angstroms light and then exposed to vapors of Freon 113 for 30seconds where upon the imagewise charged areas migrate to the substratein image configuration.

EXAMPLE VII

An imaging member is produced by preparing a softenable materialcomprising about a 12 weight percent solution of a custom synthesizedabout 80/20 mole percent copolymer of styrene and hexylmethacrylatehaving a molecular weight of about 41/300, intrinsic viscosity intoluene at about 25° C. of about 0.16. The softenable material is thencoated onto an aluminized Mylar substrate with a gravure roller and thenallowed to dry. The softenable material has a thickness of approximately2 microns. The surface of the softenable material is then coated withamorphous selenium by the vacuum deposition process as fully describedin copending application Ser. No. 813,345, filed Apr. 3, 1969. Vacuumevaporation of selenium onto the softenable layer results in an layer ofparticulate selenium being formed, having average particle size of about0.7 micron.

The member is charged negatively to 300 volts and then exposed toactivating electromagnetic radiation of about 5 ergs/cm² of 4,000angstroms light through a photographic transparency in contact with themember. The member is then heated to 80° C. for 1 second using a hotplate where upon the latent image is set against any change bysubsequent light exposure. The member is then placed upon a microscopestage and heated by radiant energy from the substage of the microscopeand while heating the member a migration image is observed being formed.The migration material migrates in image configuration forming an imagecomprising a color change from a red-orange, the color of the originalmember, to a blueish-orange the color of the imagewise areas whichmigrate.

The member is recharged to positive 100 volts and placed upon themicroscope stage and heated by radiant energy from the substage of themicroscope. The member is heated until all of the image which had beenformed is erased, i.e., imagewise migrated back to the backgroundmaterial with no visible image present. It is observed that after a fewseconds of heating the blueish-orange color is removed and thered-orange color of the original member is restored and the image iserased.

EXAMPLE VIII

The same member and process steps as described in Example VII arecarried out in this Example with the additional steps of recharging themember a second time negatively to 300 volts and then exposed toactivating electromagnetic radiation of about 5 ergs/cm² of 4,000angstroms light through a photographic transparency in contact with themember. The member is then heated to 80° C. for 1 second using a hotplate where upon the latent image is set against any change bysubsequent light exposure. The member is then placed upon a microscopestage and heated by radiant energy from the substage of the microscopeand while heating the member a migration image is observed being formed.The migration material migrates in image configuration forming an imagecomprising a color change from a red-orange, the color of the originalmember, to a blueish-orange, the color of the imagewise areas whichmigrate.

The member is recharged to positive 100 volts and placed upon themicroscope stage and heated by radiant energy from the substage of themicroscope. The member is heated until all of the image which had beenformed is erased, i.e., imagewise migrated back to the backgroundmaterial with no visible image present. It is observed that after a fewseconds of heating the blueish-orange color is removed and thered-orange color of the original member is restored and the image iserased. No evidence of the previous image is observed.

EXAMPLE IX

An imaging member is produced by preparing a softenable materialcomprising about a 12 weight percent solution of a custom synthesizedabout 80/20 mole percent copolymer of styrene and hexylmethacrylatehaving a molecular weight of about 41/300, intrinsic viscosity intoluene at about 25° C. of about 0.16. The softenable material is thencoated onto an aluminized Mylar substrate with a gravure roller and thenallowed to dry. The softenable material has a thickness of approximately2 microns. The surface of the softenable material is then coated withamorphous selenium by the vacuum deposition process as fully describedin copending application Ser. No. 813,345, filed Apr. 3, 1969. Vacuumevaporation of selenium onto the softenable layer results in an layer ofparticulate selenium being formed, having average particle size of about0.7 micron.

The member is then negatively charged to 300 volts, imagewise exposed toactivating electromagnetic radiation, light, of approximately 5ergs/cm², at 4,000 angstroms. The member is then heated to 80° C. forapproximately 2 seconds. A slight imagewise color change is observed inwhich the imagewise migrated exposed areas are blueish-orange in colorwhile the unexposed, unmigrated areas remain red-orange in color.

The member is recharged to positive 100 volts and heated at 80° C. forabout 10 to 20 seconds. The image is erased whereby the original filmcolor is observed thereby erasing the imagewise blueish-orange color.The imagewise areas returned to a red-orange color with no evidence of aprevious image being formed.

EXAMPLE X

The same member and process steps as disclosed in Example IX are carriedout in this Example with the additional steps of recharging the member asecond time negatively to 300 volts and imagewise expose to activatingelectromagnetic radiation, light, of approximately 5 ergs/cm², at 4,000angstroms. The member is then heated to 80° C. for approximately 2seconds. A slight imagewise color change is observed in whih theimagewise migrated exposed areas are blueish-orange in color while theunexposed, unmigrated areas remain red-orange in color.

The member is recharged to positive 100 volts and heated at 80° C. forabout 10 to 20 seconds. The image is erased whereby the original filmcolor is observed thereby erasing the imagewise blueish-orange color.The imagewise areas returned to a red-orange color with no evidence ofany previous images being formed.

Local erasure is demonstrated in Examples VII and VIII in which themicroscope is used for local radiant heating to locally develop a memberin which the image was set by slight heating. These Examples demonstratethat local erasure of an imaged or non-imaged member by a technique suchas local radiant heating can be performed. Therefore, a member may belocally erased or images reformed locally so that a member containing alarge volume of material may be updated, changed, corrected by thislocal image erasure and local reformation. Local heating could be doneby any suitable means including very localized radiation from lasers.The imaged or non-imaged areas outside of the locally heated areas arenot effected.

What is claimed is:
 1. A process for erasing a migration image patterncomprising: (a) providing an image member comprising a layer ofsubstantially electrically insulating softenable material and a layer ofmagnetizable migration material distributed as discrete particles eachsmaller than the migration image pattern in depth in said softenablematerial in image pattern configuration and comprising in addition tosaid image pattern of migration material a complementary backgroundimage layer of migration material in said softenable material and spacedapart, in depth, from said image pattern, said migration material beinginsoluble in said softenable material, said softenable material capableof being softened sufficiently to allow migration of said migrationmaterial in said softenable layer; (b) applying an electrical ormagnetic migration force to the migration material sufficient to causethe image pattern of the migration material to migrate to thecomplementary background image layer of migration material; and (c)softening the softenable layer sufficient to allow migration of themigration material whereby the image pattern layer of migration materialmigrates to the complementary background image layer of migrationmaterial thereby erasing the image pattern and forming a reusablemigration imaging layer.
 2. A process for erasing a migration imagepattern comprising: (a) providing an image member comprising a layer ofsubstantially electrically insulating softenable material and a layer ofmigration material distributed as discrete particles each smaller thanthe migration image pattern in depth in said softenable material inimage pattern configuration and comprising in addition to said imagepattern of migration material a complementary background image layer ofmigration material in said softenable material and spaced apart, indepth, from said image pattern, said migration material being insolublein said softenable material, said softenable material capable of beingsoftened sufficiently to allow migration of migration material in saidsoftenable layer, wherein said softenable layer contains an overlayer ofsubstantially electrically insulating material capable of retaining anelectrostatic charge which is sufficient to cause migration to occurwhen the softenable material is softened sufficient to allow migration,said charge being retained on its surface for about 0.1 to about 30seconds subsequent to the beginning of step (c); (b) applying anelectrical or magnetic migration force to the migration materialsufficient to cause the image pattern of migration material to migrateto the complementary background image layer of migration material; and(c) softening the softenable layer sufficient to allow migration of themigration material whereby the image pattern of migration materialmigrates to the complementary background image layer of migrationmaterial thereby erasing the image pattern and forming a reusablemigration imaging layer.
 3. A process for erasing a migration imagepattern comprising: (a) providing an image member comprising a layer ofsubstantially electrically insulating softenable material and a layer ofmigration material distributed as discrete particles each smaller thanthe migration image pattern in depth in said softenable material inimage pattern configuration and comprising in addition to said imagepattern of migration material a complementary background image layer ofmigration material in said softenable material and spaced apart, indepth, from said image pattern, said migration material being insolublein said softenable material, said softenable material capable of beingsoftened sufficiently to allow migration of migration material in saidsoftenable layer, wherein said softenable layer contains an overlayer ofmaterial selected from the group consisting of p-tertiary butyl phenolformaldehyde resin and p-phenol phenol formaldehyde resin; (b) applyingan electrical or magnetic migration force to the migration materialsufficient to cause the image pattern of migration material to migratetoward the complementary background image area of migration material andsaid overlayer of material; and (c) softening the softenable layersufficient to allow migration of the migration material whereby theimage pattern of migration material migrates toward the complementarybackground image layer of migration material and said overlayer ofmaterial thereby erasing the image pattern and forming a reusablemigration imaging layer.
 4. A process for erasing a migration imagepattern comprising: (a) providing an image member comprising a layer ofsubstantially electrically insulating softenable material and a layer ofmagnetizable migration material distributed as discrete particles eachsmaller than the migration image pattern in depth in said softenablematerial in image pattern configuration and comprising in addition tosaid image pattern of migration material a complementary backgroundimage layer of migration material in said softenable material and spacedapart in depth, from said image pattern, said migration material beinginsoluble in said softenable material, said softenable material capableof being softened sufficiently to allow migration of migration materialin said softenable layer, said softenable layer containing an overlayerof material; (b) applying an electrical or magnetic migration force tothe migration material sufficient to cause the image pattern ofmigration material to migrate towards the complementary background imagelayer of migration material and overlayer of material; and (c) softeningthe softenable layer sufficient to allow migration of the migrationmaterial whereby the image pattern layer of migration material migratestowards the complementary background image layer of migration materialand overlayer of material thereby erasing the image pattern and forminga reusable migration imaging layer.
 5. A process for erasing a migrationimage pattern comprising: (a) providing an image member comprising alayer of substantially electrically insulating softenable material and alayer of migration material distributed as discrete particles eachsmaller than the migration image pattern in depth in said softenablematerial in image pattern configuration and comprising in addition tosaid image pattern of migration material a complementary backgroundimage layer of migration material in said softenable material and spacedapart, in depth, from said image pattern, said migration material beinginsoluble in said softenable material, said softenable material capableof being softened sufficiently to allow migration of migration materialin said softenable layer, wherein the softenable material contains asurface skin located at the entire free surface area on the side of theimaging member closest to the background of migration material of saidsoftenable layer, formed by exposing said free surface to hardeningradiation sufficient to form said surface skin having a thickness in therange not greater than about 0.3 micron and having a viscosity duringdevelopment of said member greater than said viscosity of the softenablematerial of said softenable layer; (b) applying an electrical ormagnetic migration force to the migration material sufficient to causethe image pattern of migration material to migrate towards thecomplementary background image layer of migration material and saidsurface skin; and (c) softening the softenable layer sufficient to allowmigration of the migration material whereby the image pattern layer ofmigration material migrates towards the complementary background imagelayer of migration material and said surface skin thereby erasing theimage pattern and forming a reusable migration imaging layer.
 6. Theprocess according to claim 5 wherein said hardening radiation isultraviolet radiation.
 7. The process according to claim 5 wherein saidsurface skin has a thickness of not greater than about 0.2 micron. 8.The process according to claim 5 wherein said surface skin issubstantially electrically insulating and capable of retaining anelectrostatic charge which is sufficient to cause migration to occurwhen the softenable material is softened sufficient to allow migration,said charge being retained on its surface for about 0.1 to about 30seconds subsequent to the beginning of step (c).
 9. A process accordingto claim 5 wherein said electrical force comprises forming anelectrostatic force on said member.
 10. A process according to claim 9wherein said reverse migration force is an attraction of said imagewisemigrated migration material to charges of a polarity opposite to thepolarity of charges on the imagewise migrated migration material, saidopposite polarity charges at a location spaced apart from said imagewisemigrated migration material thereby causing the imagewise migratedmigration material to migrate to the background of migration material.11. A process according to claim 5 wherein said migration material ismagnetizable.
 12. The process according to claim 5 wherein the migrationmaterial is photosensitive.
 13. The process according to claim 12wherein the migration material is photoconductive.
 14. A process forerasing a migration image pattern comprising: (a) providing an imagemember comprising a layer of substantially electrically insulatingsoftenable material and a layer of migration material distributed asdiscrete particles each smaller than the migration image pattern indepth in said softenable material in image pattern configuration andcomprising in addition to said image pattern of migration material acomplementary background image layer of migration material in saidsoftenable material and spaced apart, in depth, from said image pattern,said migration material being insoluble in said softenable material,said softenable material capable of being softened sufficiently to allowmigration of migration material in said softenable layer, wherein saidsoftenable material comprises a single layer wherein a portion of thesoftenable layer comprises a surface skin located on the side of theimaging member closest to the background of migration material having achemical composition different from the bulk of the softenable materialand having a thickness not greater than about 0.3 micron and a viscosityduring development of the member greater than that of the bulk of saidsoftenable material; (b) applying an electrical or magnetic migrationforce to the migration material sufficient to cause the image pattern ofmigration material to migrate towards the complementary background imagelayer of migration material and said surface skin; and (c) softening thesoftenable layer sufficient to allow migration of the migration materialwhereby the image pattern of migration material migrates towards thecomplementary background image area of the migration material and saidsurface skin thereby erasing the image pattern and forming a reusablemigration imaging layer.
 15. The process according to claim 14 whereinsaid surface skin is a phenol formaldehyde resin.
 16. The processaccording to claim 15 wherein said softenable material is a mixture ofpolystyrene and acrylic polymers.
 17. The process according to claim 16wherein said phenol formaldehyde resin is selected from the groupconsisting of p-tertiarybutyl phenol formaldehyde resin and p-phenylphenol formaldehyde resin and said softenable layer is a mixture ofstyrene and hexylmethacrylate.
 18. The process according to claim 14wherein said surface skin is substantially electrically insulating andcapable of retaining an electrostatic charge which is sufficient tocause migration to occur when the softenable material is softenedsufficient to allow migration of the migration material said chargebeing retained on its surface from about 0.1 to about 30 secondssubsequent to the beginning to step (c).